DEVELOPMENT OF INDIA'S FIRST COUPLING TRANSFORMER · PDF fileDC Flux Density Shift in Transformer Core Flux density shifts in the core caused by I DC The magnitude of

February 22nd - 23rd , 2017, Scope Complex, Lodhi Road, New Delhi, India

6th International Conference on Large Power Transformers – Modern Trends in

Application, Installation, Operation & Maintenance

by

DEVELOPMENT OF INDIA'S FIRST COUPLING

TRANSFORMER FOR STATCOM APPLICATIONS

NANDHAKUMAR RAJARAM

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[email protected]

CROMPTON GREAVES LIMITED

MUMBAI-INDIA

By




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Outline of Presentation

STATCOM

Necessity Of STATCOM Coupling Transformer

STATCOM Coupling Transformer Specifications

Design Features Of Coupling Transformer

DC Current and Harmonics

Overfluxing

Effect of Core Joints and Core Hotspot Temperature

Winding Hot Spots

DC Effects on Flitch Plates, Tank, Thermal and Acoustic Noise

Factory Testing

Conclusion




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STATCOM

STATCOM is the state of art of technology in the reactive power system

It is IGBT based Voltage Source Converter (VSC)

VSC working is based on SPWM techniques

It has the capability to generate and absorb variable reactive power

Specifically meant for Reactive Power Compensation of Highly Dynamic Loads

STATCOM does not require any passive components like capacitor or reactor for

generating or absorbing reactive power

X

X

X

V s V

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C R

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B

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Mode-1: VSTAT < VS, STATCOM absorbs reactive power (Inductive)

Mode-2: VSTAT > VS, STATCOM injects reactive power (Capacitive)

Mode-3: VSTAT = VS, STATCOM does not work (Zero current)

Vs - Network Voltage

VSTAT - STATCOM Output Voltage

VSC - Voltage Source Converter

Single Line Diagram of STATCOM




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STATCOM - Features

Smooth and Ultra Dynamic Stepless Response

Continuously maintain unity Power Factor (P.F.) within its dynamic range

Dynamic range is in capacitive as well as inductive mode based on system demand

Reactive power is generated in IGBT based VSC and therefore does not require any

passive elements

No harmonics generation for adoption of SPWM technique

Integrated Active Harmonic Filter

Suited to a hybrid solution of STATCOM plus switched capacitors

Cheaper than SVC below 50 MVAr

Easy to install, less maintenance & Environment friendly

TSC

t

+ Q

- QSTATCOM




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Necessity Of STATCOM Coupling Transformer

Worldwide available STATCOM system voltage is less than 40 kV

STATCOM cannot be directly connected to the high voltage grids like 220kV, 400 kV, etc.,

Coupling Transformer is the intermediate device to connect STATCOM system and high

voltage grid with bidirectional power flow

The primary side of coupling transformer is connected to high voltage grid and secondary

side is connected to the STATCOM system

Coupling transformer does not require any tap changer since its voltage is controlled by

the STATCOM system

Coupling transformer step-up duty is too severe in STATCOM application and it must be

specifically designed to meet those requirements




Rated Power (MVA) HV / LV 33.334 / 33.334

Rated No Load Voltage (kV) HV / LV (400/√3) / 20

Rated Frequency (Hz) & No. of Phase 50, Single Phase

Type Of Core Construction 3 Limb Core (1 Wound & 2 Return Limbs)

Vector Group YNd1 (3 Phase Bank)

% Impedance at rated MVA base HV – LV 12.5% with +/-5% Tolerance

Duty continuous 100% Capacitive & Inductive Reactive Loading

Winding Insulation Levels

Full Wave Lightning Impulse Voltage (kVp)

HV/LV/Neutral1425 / 170 / 170

Power Frequency (kVrms) HV / LV / Neutral 630 / 70 / 70

Switching Impulse Voltage (kVp) HV / LV 1050 / --

Type of Cooling and Arrangement ONAF2 and 2 x 50%

Name Of Project400/220 kV Substation at NP Kunta, Andhra Pradesh,

India

STATCOM Coupling Transformer Specifications

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Design Features Of Coupling Transformer

Coupling transformer must be designed and rated to carry 100% dynamic reactive power

loadings

Due to high frequency switching of IGBT and the DC capacitor voltage, STATCOM produces

DC current and harmonics in the secondary side of coupling transformer.

Certain level of direct current and harmonic currents consistent with the most onerous

operation of STATCOM should be withstood by the coupling transformer without loss of life

Presently very few manufacturers worldwide have made coupling transformers for 400 kV

system voltages

The main design features of the coupling transformer to be considered are


Overfluxing


Winding Hot Spots


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DC Flux Density Shift in Transformer Core

Flux density shifts in the core caused by IDC

The magnitude of ФDC depends on the magnitude of the dc, No. of turns in the windings

carrying IDC & reluctance path of ФDC

The result is the ФDC adds to the ФAC in positive half-cycle and subtracts from the ФAC in

the negative half–cycle

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Half–Cycle Saturation of Core under Effect of DC Current

When the ФDC is max or transformer design “B” is high, the core peak “B” moves to the magnetic

core pre-saturation range in the positive half cycle, resulting in core saturation for a small part of a

cycle is called half-cycle or part-cycle saturation

For higher magnitudes of dc, the core provides a much higher reluctance to the DC Ampere-Turns

It results in a smaller incremental increase in the flux density shift and a higher peak magnetizing

current pulse

B – H characteristics of the coupling transformer core materials is inherently extremely non-linear

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Overfluxing

Voltage / Frequency ratio is called overfluxing factor

V/F ratio is generally constant, if “V” increases above nominal & “F” decreases

below nominal, core reach under saturation

Designing of minimum flux density for coupling transformer

Incremental flux density due to DC current

Frequency variation

Coupling transformer flux density variation due to STATCOM system

Capacitive loading, leading to increase in flux density in the core

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Mitred joint has a greater reluctance than a step-lap joint

Core magnetic material & core joint type have some influence on core saturation characteristics. However

this influence is small & depends on the core type & the operating flux density in the core

Typical temperature distribution in the core considering the effect of dc and harmonics of the STATCOM

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Winding Hot Spots

Winding hot spots are determined with the help of eddy loss

distribution which is governed by leakage field plot

The flux lines are predominantly axial in the central part

At the winding ends, due to fringing flux, the flux lines tend to

bend radially & there will be a radial component of eddies in

addition to axial eddies

Due to natural convection, temperature at top part will be higher

F. O probes are located within the LV & HV windings, on

transformer core and oil

The probes are to be monitored throughout testing process and

services

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Part-cycle of core saturation operating region as the result of dc

bias will produce greater levels of acoustical noise & tank

vibrations

Noise can be reduced by lowering “B” & by other means

IDC increases, the magnetic “B” in the tie–plates increases linearly until it reaches the

magnetic saturation level of the tie–plate material

To reduce tank stray losses, CRGO magnetic shunts are provided on the shorter side of HV

& LV windings of the tank

The height of CRGO magnetic shunts should be higher than the height of windings

Higher eddy & circulating current losses in the windings & the structural parts of the coupling

transformer; increasing their temperatures, which will require extra cooling

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Factory Testing

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CONCLUSIONThis coupling transformer technology, development, design and manufacturing is

based on Make in India concept and sell them anywhere in the world

Coupling Transformer should be designed and withstand the behavior of STATCOM

as mentioned below

Dynamic reactive power variation from capacitive to inductive & vice versa

DC and harmonics generated by STATCOM

Incremental flux density due to dc current

Significant rise of losses and system voltage instability

Increased reactive power consumption

The rise of internal hotspots, increased noise levels and tank vibrations are the effects

of STATCOM on the coupling transformer

The successful design, manufacturing process, factory testing and installation of this

400 kV coupling transformer will establish a “future-proofing” milestone for technology

development in India in the field of high voltage equipments for DRPC applications




REFERENCES

[1] Ramsis Girgis & Kiran Vedante: “Effect of GIC on Power Transformers and Power Systems”, Presented at the

IEEE T&D Conference, May 2012, Orlando, FL.

[2] P. Picher, L. Bolduc et al, “Study of the acceptable DC current limit in core-form Power Transformers”, IEEE

Transactions on Power Delivery, Vol. 12, No. 1, January 1997, Page 257-265.

[3] Ramsis Girgis & Chung-Duck KO, “Calculation Techniques And Results Of Effects Of GIC Currents as Applied

to Two Large Power Transformers”, IEEE Transactions on Power Delivery, Vol. 7, No. 2, April 1992, Page 699-

705.

[4] P.R. Price. “Geomagnetically induced current effects on transformers” (IEEE Transactions on Power Delivery,

vol. 17, 2002, p. 1002 – 1008).

[5] S.Jeevananthan, R. Nandhakumar and P. Dananjayan, “Inverted sine Carrier for fundamental fortification in

PWM Inverters and FPGA based Implementations”, Serbian Journal of electrical engineering, Vol.4, No.2,

November 2007, Page(s) 171-187.

[6] R. Nandhakumar & S.Jeevananthan, “Inverted Sine Carrier Pulse Width Modulation for Fundamental

Fortification in DC-AC Converters”, Power Electronics and Drive Systems, 2007, PEDS’2007, Seventh

International Conference (IEEE), Bangkok, Thailand, Page(s): 1028 – 1034.

[7] CIGRE Working Group B4.53 report, “Guidelines for the procurement and testing of STATCOMS”. CIGRE

Brochure No. 663, August 2016.

[8] WG 12.06 Large transformers “Final report of WG 06 of Study Committee 12 (Transformers)”. ELECTRA No.

82, May 1982, pp. 31 – 46.

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Thank you

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QUESTIONS ???

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DEVELOPMENT OF INDIA'S FIRST COUPLING TRANSFORMER · PDF fileDC Flux Density Shift in Transformer Core Flux density shifts in the core caused by I DC The magnitude of